Enhancing the Data Reliability of Multilevel Storage in Phase Change Memory with 2T2R Cell Structure

Multilevel storage and the continuing scaling down of technology have significantly improved the storage density of phase change memory, but have also brought about a challenge, in that data reliability can degrade due to the resistance drift. To ensure data reliability, many read and write operation technologies have been proposed. However, they only mitigate the influence on data through read and write operations after resistance drift occurs. In this paper, we consider the working principle of multilevel storage for PCM and present a novel 2T2R structure circuit to increase the storage density and reduce the influence of resistance drift fundamentally. To realize 3-bit per cell storage, a wide range of resistances were selected as different states of phase change memory. Then, we proposed a 4:3 compressing encoding scheme to transform the output data into binary data states. Therefore, the designed 2T2R was proven to have optimized storage density and data reliability by monitoring the conductance distribution at four time points (1 ms, 1 s, 6 h, 12 h) in 4000 devices. Simulation results showed that the resistance drift of our proposed 2T2R structure can significantly improve the storage density of multilevel storage and increase the data reliability of phase change memory.